Method of screening a color image reproducer

ABSTRACT

The faceplate section of a tri-color cathode-ray tube of the shadow mask type is coated with a photosensitive layer and is simultaneously exposed through apertures of the shadow mask with actinic energy from six separate energy sources symmetrically located relative to a reference position which simulates the source of one of the three electron beams of the tube. For example, in screening with green phosphor, the light sources are symmetrically positioned relative to a reference position which simulates the source of the electron beam assigned to excite green phosphor in the operation of the tube.

1451 Oct. 23, 1973 METHOD OF SCREENING A COLOR IMAGE REPRODUCER [75]Inventor: Sam l-l. Kaplan, Chicago, Ill.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

22 Filed: Sept. 13, 1971 21 Appl. No.: 179,921

Primary Examiner-Norman G. Torchin Assistant Examiner-Edward C. KimlinAttorney-Pederson John J. and Corneluis J. OConnor [57] ABSTRACT Thefaceplate section of a tri-color cathode-ray tube of the shadow masktype is coated with a photosensitive layer and is simultaneously exposedthrough apertures of the shadow mask with actinic energy from sixseparate energy sources symmetrically located relative to a referenceposition which simulates the source of one of the three electron beamsof the tube. For example, in screening with green phosphor, the lightsources are symmetrically positioned relative to a reference positionwhich simulates the source of the electron beam assigned to excite greenphosphor in the operation of the tube.

8 Claims, 8 DrawingFigures PATENIEnum 2 3 ma Wang Q s m H FwQMK/Q 1METHOD OF SCREENING A COLOR IMAGE REPRODUCER BACKGROUND OF THE INVENTIONThe present invention is directed to screening a color cathode-ray tubewith a plurality, usually three in number, of different phosphors. Whileof general application, it concerns most particularly screening of tubesin which the phosphor deposits are desired to be smaller than thetransparentportions of the color-selection electrode through whichelectron beams are permitted to impinge upon assigned ones of thevarious phosphor materials in the synthesizing of an image in simulatednatural color.

A particular need for the subject invention is in the production oftri-color cathode-ray tubes in which the screen is a mosaic ofphosphor-dot deposits defining a multiplicity of dot triads each ofwhich includes a dot of green, a dot of red and a dot of blue phosphor.The shadow mask of such a tube has essentially circular apertures withone aperture in juxtaposition relative to each phosphor dot triad sothat three electron beams generated from a gun cluster pass through theholes of the mask, as the beams are scanned by the usual deflectionfields, and arrive at the screen in such directions that each beamexcites only the color phosphor to which it has been assigned. In onetype of tube currently' produced commercially, it is highly desirablethat the phosphor dots be smaller in size than the electron beams. Thisis true, for example, in the so-called black surround type of tubewherein each phosphor dot is surrounded by graphite or some otherlight-absorbing material for enhancement of both brightness and contrastas described in U.S. Pat. No. 3,146,368, issued Aug. 25, 1964 to JosephP. Fiore et al. and assigned to the assignee of the present invention. Asimilar size relation of phosphordot to electron beam is desirable forpost-'deflection-focus or post-deflection-acceleration color tubeswherein the electron beams are subject to a focusing field after passingthrough the plane of deflection.

An attractive method for screening such tubes to achieve phosphor dotssmaller in size than the apertures of the mask is described and claimedin applicants aforeidentified copending application. In accordance withthe disclosure of that application, when the screen is being processedwith one particular phosphor, such as green, by means of photographicprinting, a photosensitive slurry coating is exposed twice, once withthe exposing light source positioned to simulate e e r n g n f thelu andOn -th he? posing light source positioned to simulate the blue electrongun of the tube. These exposures occur sequentially and, in effect, theyexpose elemental areas of the screen that totally surround eachelemental area that is to receive a deposit of green phosphor. Theexposure ;time is long enough that the change in solubility of the 5coating resulting from the two exposures leaves a series of unexposedelemental areas that individually are smaller than the apertures of themask. These unexposed areas are latent images of the green phosphordeposits and are developed in the usual way. Repeating this process foreach of the three colors of the screen provides the desired screenstructure with phosphor dots smaller in area than the apertures of theshadow mask. This is an acceptable screening process but is improvedthrough the present invention by permitting the W two exposures to takeplace simultaneously rather than sequentially.

Another screening disclosure relevant to the subject invention is foundin U.S. Pat. No. 3,152,900, issued Oct. 13, 1964 to P. E. Kaus et al. Italso concerns preparing phosphor dot deposits on the screen of a colortube with an area that is less than the area of the apertures in theshadow mask through which photographic screening takes place. Inachieving its objective, the process of the patent utilizes a ring orannular type light source in conjunction with a positive resist by whichis meant a photosensitive material which is rendered solu- :ble in agiven solvent in response to exposure to actinic energy, which usuallyis ultraviolet light. While the disiclosure permits photographicallyprinting phosphor dots of the desired dimension relative to the crosssection of the electron beams of the tube, it is confined to formingdots of circular configuration. In contradistinction, both the processof applicants aforementioned copending application and that describedherein gphosphor depos it is approximately 10 percent larger in areathan the circular dot of Kaus et al. with theTPQlfiHfiidiFiQLlbfiBWPQfi? the eme -H i Accordingly, it is an object ofthe invention to pro- .vide a novel method of coating the screen of acolor image reproducer, such as a cathode-ray tube, with a ipattern ofat least three different phosphor materials.

It is a further object of the invention to improve photoprinting ofcolor'cathode-ray tube screens by multi- 4 ple simultaneous exposures ofelemental screen areas.

It is a particular object of the invention to provide a Q'screeningmethod to attain elemental phosphor deposits smaller in size than thetransparent portions of the fcolor-selection electrode in much the samefashion as the above-mentioned copending application but featuringsimultaneous as distinguished from sequential exposures from a pluralityof energy sources.

More specifically, it is an object of the invention to' provide a novelmethod of screening a shadow mask type of color tube with phosphor dotsof essentially hexagonal configuration and smaller in area than theapertures of the shadow mask.

The method of the invention in its broadest aspect is for coating thescreen of a color image reproducer with at least three differentphosphor materials arranged in an interlaced pattern with a deposit ofany one phosphor surrounded by like deposits of the remaining phosphorsfor selective energization by at least three electron beams havingaccess to the phosphor deposits through transparent portions of acolor-selection electrode, such as a shadow mask. The method comprisesthe step of forming over the screen a layer of a material having asurface characteristic that is subject to change in response toimpingement by actinic energy. Thereafter elemental areas of the layerare simultaneously exposed through transparent portions of thecolorselection electrode with actinic energy from an even number ofseparate energy sources symmetrically located relative to a referenceposition which simulates the source of the one of the electron beams ofthe tube to create in the layer a latent image. Finally, that latentimage is developed.

In one aspect of the invention for screening a tricolor tube having ashadow mask with circular apertures, a series of six light sources aresymmetrically positioned about a reference point simulating the positionof the electron gun assigned to excite the phosphor in process.Simultaneous energization of the six light sources exposes six elementalareas of the screen which, in processing green phosphor by way ofillustration, constitute the areas that are to receive red and bluephosphors. These surrounding elemental areas completely encircle an areaassigned to receive green phosphor and conjointly reduce the unexposedelemental area in size so that it is smaller than the apertures of theshadow mask.

DESCRIPTION OF THE DRAWINGS The features of the present invention whichare believed to be novel are set forth with particularity in theappended claims. The invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings, in theseveral figures of which like reference numerals identify like elements,and in which:

FIG. 1 is an enlarged showing of a fragmentary portion of a color tubescreen;

FIG. 2 is a schematic representation ofa screen in an exposure position;

FIGS. 3-5 are illustrative exposure diagrams;

FIG. 6 represents an exposure light source;

FIG. 7 shows a fragment of an aperture mask; and

FIG. 8 is a light source arrangement for exposing a screen through amask of the type shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularlyto FIG. 1, the arrangement there represented is an enlarged fragmentaryportion of the screen of a tri-color cathode-ray tube. It is formed onthe faceplate or cap portion of the envelope which is normally separatefrom the remaining part of the envelope structure to facilitatescreening. The faceplate is processed by first forming over the screen alayer or coating of a material having a surface characteristic that issubject to change in response to impingement by actinic energy. A choiceis available, depending on the nature of the screening process to beemployed. In one process the screen is coated with a photosensitivematerial or resist having a solubility in a solvent that is changed byexposure to actinic energy. In another, the screen is coated with aconductive layer and then with a superposed photoconductive layer whichis uniformly charged and thereafter selectively discharged in responseto the impingement of light. For convenience, let it be assumedinitially that the layer applied to the cleansed faceplate as the firststep of the screening process is a photosensitive material. Thesematerials are characterized as either positive or negative. The positivetype has the characteristic that is rendered soluble in a solvent uponexposure to ultraviolet light whereas otherwise the material isinsoluble. The negative type, on the other hand, is normally soluble ina particular solvent but is rendered insoluble in response to suchexposure. The process under consideration may be advantageously utilizedwith either type material but, again for convenience, it will be assumedthat the coating layer is a positive photosensitive resist, such as oneof AZ resists available from the Shipley Company of Newton,Massachusetts. It is also convenient to consider that the photosensitivecoating material includes a phosphor in particulate form as aningredient. Since the screen is to be coated with green, blue and redphosphors, let it be further assumed that the green phosphor is to beapplied first and is an ingredient of the photoresist coating.

After the entire screen area has been covered with a layer of suchmaterial and after that layer has been permitted to dry, the screen isexposed through transparent portions of the color-selection electrode.In other words, the screen is exposed by directing actinic energy to thecoating layer through the apertures of the shadow mask. For the assumedconditions it is necessary to expose the entirety of the screen exceptthe elemental areas thereof assigned to the particular phosphor materialin process, green for the case at hand. In other words, the exposurestep requires that the elemental areas of the screen assigned to receiveblue and the elemental screen areas assigned to red be exposed, leavingunexposed only the set of elemental screen areas intended to receivegreen phosphor.

In practicing the prior art, specifically that of the aforementionedcopending application, this exposure is accomplished in two steps inconventional exposure chambers or lighthouses having a small area orpoint source of ultraviolet light and a suitable collimator by means ofwhich the light is directed to the screen through the apertures of theshadow mask. In the first exposure step, the light source is positionedto simulate the electron gun of the tube assigned to excite the bluephosphor which results in exposure of all elemental areas of the screenthat are to receive a deposit of blue phosphor. In a similar but secondexposure step, the light source is positioned to simulate the electrongun assigned to excite the red phosphor so that all elemental areas ofthe screen to receive red phosphor are exposed.

The conditions of the screen at this juncture will be readily understoodby reference to FIG. 1. Each of the circles designated B represents animage projected on the coated screen with the light source simulatingthe beam assigned to excite the deposits of blue phosphor. Similarly,the circles designated R indicate the images projected on the screenthrough the shadow mask with the light source simulating the gunassigned to energize the red phosphor. It will be observed that eachcircle or elemental screen area designated G is completely surrounded bya series of six similar elemental areas alternately designated R and B.The dash-dot circles around each of these latter areas represent thatthe exposed elemental areas R and B, giving due regard to the penumbraeffects of the light source, which are suffciently large if the exposuretime is adequately long to partially overlap the elemental area G thatthe exposed areas R & B surround. Collectively, they cause thenonoverlapped portion of the surrounded elemental area G to have ahexagonal configuration with the hexagon enscribed within the circle G.In short, the described sequential exposure steps leave unexposedcusp-shaped areas G which are smaller in size than the apertures of theshadow mask. The mask apertures are essentially the same size as thecross-hatched circles R and B. All elemental areas of the screendesignated G concurrently receive a similar treatment, that is to say, aset of cusp-shaped hexagonal areas G distributed over the screen areaconstitute the unexposed elemental areas and represent a latent image ofthe set of elemental areas to be developed in green phosphor. This istrue even though the drawing has been simplified to particularize ashowing of the unexposed cusp hexagon G for only a single elementalscreen area located centrally of the fragmentary portion illustrated inFIG. 1. If the composition of the coating area be Shipley resist AZlllin the amount of 75 per cent, having green phosphor suspended therein inthe amount of 25 per cent, an expos'ure interval in the order of 8minutes will produce the desired small sized cusp-shaped hexagonal greenphosphor dots.

The next step of the process is to develop in the green phosphor theunexposed elemental areas G. This is done simply by washing the screenin a solvent for the photosensitive coating which removes the exposedareas B and R because they are soluble in the solvent whereas theunexposed portions G are insoluble. The process as thus far described isthe same as that of the applicants copending application and in likefashion deposits of blue and red phosphors, smaller in size than theapertures of the shadow mask may be applied to the screen. It should benoted in passing that, after the application of the first two sets ofphosphor dots, each such set should be treated so that its resistcomponent is rendered insensitive to actinic energy lest one set ofphosphor dots be destroyed in processing the next succeeding set. Forexample, after developing the green phosphor dots, the screen is coatedwith a photosensitive resist including blue phosphor and is then exposedfrom the green and red positions to create latent images of the bluedots. If the previously applied green phosphor dots retain theirphotosensitivity, they will wash off in developing the blue dots. Thismay be avoided by treating the green phosphor dots with a desensitizingchemical or protecting them with a barrier layer before laying down theblue phosphor slurry. A resin that is not soluble in the solvent of theslurry resist is suitable; nitrocellulose is an acceptable material touse for the barrier layer.

The significant processing change introduced by the subject invention isthat the multiple exposures employed in developing a single phosphor aremade simultaneously from at least two, but preferably more, separatesources of actinic energy. In order to permit such simultaneousexposure, it is necessary to position the energy sources symmetricallyrelative to a reference position which simulates the source of the gunassigned to excite the phosphor in process. This is a distinct departurefrom prior practices as will be readily understood by reference to theschematic diagram of FIG. 2 which illustrates the more conventionalsetup of the exposure chamber. The subassembly of a screen 10 with itsshadow mask 11 installed in position is supported on the lighthouse forexposure from a light source L which is usually a mercury arc lamp orother ultraviolet light generator. In the conventional lighthouse sourceL is positioned off the axis of the lighthouse by a distance S and at adistance p from the mask which, in turn, is spaced from the screen by anamount q. The spacing S is chosen so that the light source L simulatesthe electron gun assigned to excite the particular phosphor that isbeing applied. This is the appropriate position for source L if thephosphor is included as an ingredient of a photosensitive layer of thenegative type. When the photosensitive material is positive and phosphordepos-' its smaller in area than the mask apertures are desired, oneexposes the red and blue elemental areas in processing green phosphor,as described above, rather than exposing the set of green elementalareas. One approach that suggests itself is indicated in FIG. 3 wherethe triangle interconnects centers of one phosphortriad and where lightsources L and L are positioned in the lighthouse to project ultravioletlight on the elemental areasB and R intended to receive blue and redphosphors. This lighthouse arrangement indeed exposes the six elementalareas clustered about the single elemental area G to receive greenphosphor and would appear on the surface to accomplish the objective ofsimultaneous exposure. A practical difficulty however is encountered,one attributable to the geometry of the tube in question. Because ofsuch things as the variation in q distance with deflection angle thereis a tendency to what is known as a degrouping effect of thephotographically printed phosphor triads particularly at the edges ofthe screen. In conventional exposure techniques, processing phosphor ina negative resist, such degrouping effects are minimized by the use of acorrection lens interposed in the optical path between the light sourceand the shadow mask but each such lens is tailored to one particularposition of the light source in the lighthouse. For thiS reason, acorrection lens for exposing blue phosphor areas is inappropriate foruse with respect to a light source positioned to expose red phosphorareas. Applicant has discovered that through a different physicalarrangement of a plurality of separate light sources, simultaneousexposures from all such sources can indeed take place. One arrangement,employing only two light sources, is represented schematically in FIG. 4wherein one light source is positioned to exposed elemental screen areasB outlined in dash-dot construction lines, while the other is positionedto expose elemental screen areasR outlined in broken-construction lines.A projection of the point light sources is designated L and L which, forthis arrangement are symmetrically positioned relative to the referenceposition which simulates the green electron gun. With respect to FIG. 4,this reference position may be considered to be the center ofcusp-shaped hexagon G. With the light sources thus positioned, a singlecorrecting lens appropriate for utilization with a light sourcepositioned to simulate the green electron gun is equally efficacious fortwo light sources positioned in the manner indicated in FIG. 4.Accordingly, the light sources may be energized together to effectsimultaneous exposure of the setof elemental areas R assigned to redphosphor and the set of elemental areas 8' assigned to blue phosphor.Aside from this modification having to do with the position of the twolight sources L and L and the fact of simultaneous multiple exposures,the process is the same as described and claimed in the above-identifiedcopending application.

The inventive process is not confined to the use of two light sources,such as L, and L an advantage is realized by using a larger numberalthough there should be an even number of light sources symmetricallypositioned relative to the simulated reference position of the greengun, again, assuming for convenience that the green phosphor is beingprocessed. A preferred arrangement employs six light sources andestablishes the exposure pattern of FIG. 5 where the designations L -Lrepresent projections of the exposing light sources. All are symmetricalabout the simulated position of the green gun (the center of hexagon G)with the odd numbered sources positioned to expose elemental areas Bassigned to blue and the even numbered sources positioned to exposeelemental areas R assigned to red.

In FIG. 5 each of the exposed elemental areas B and R is enclosed inthree concentric circles represented respectively by a full constructionline, a dash construction line, and a dash-dot construction line. Thissignifies that each of the six elemental areas for the illustratedcluster represented in FIG. 5 does, in fact, receive three superimposedor simultaneous exposures which may have the added benefit of a muchfaster exposure rate. The reason that each such elemental area receivesthree such exposures will be understood by further reference to FIG. 1.

The elemental area designated B as explained above and as is evidentfrom the crosshatching, is included in a group of six similarlycrosshatched elemental areas completely surrounding elemental area 6,.Inspection of the figure shows that this same elemental area B isincluded in a like cluster of six surrounding elemental area G and isfurther included in a third such cluster surrounding elemental area GAll three of these clusters are exposed at the same time from which itis clear that elemental area B, is subjected simultaneously to threesuperimposed or registered exposures. The same is true of all elementalareas B and all elemental areas R during the processing of the greenphosphor.

As a practical matter it is not necessary to have physically separatesources of light or actinic energy; they may be derived from a commonenergy source in the manner of FIG. 6 which shows the usual ultravioletlamp backed by a reflector 21 and distributing light through a diffusionplate 22 to a series of holes or transparent areas 23 provided in aplate 24 located between lamp 20 and the aperture mask. The number oftransparent elements 23 corresponds to the number of simulated separatelight sources desired. As stated, there should be an even number whichmeans that it is convenient to employ two or four, although six ispreferred. Where six are used, the transparent portions 23 must beprovided to satisfy the geometry requirements described in relation toFIG. 5 in the discussion of light sources Is -L If desired, at eachlocation 23 of plate 24 there may be a diffusion tip to achieve a betterdistribution of light especially at the edge portions of the screen. Ofcourse, the pattern of light sources must properly relate to theaperture pattern of the mask especially if the latter is arranged tominimize moire.

It is known that moire effects may be experienced when scanning a shadowmask color tube and in order to minimize moire it is conventionalpractice to arrange the apertures of the mask to define a series ofregular hexagonal patterns individually having a major dimensionextending in the vertical direction in the manner represented in FIG. 7.This figure shows a small portion of the shadow mask including sevenapertures in which the center to center spacing of adjacent holes isdesignated W. If the center aperture is ignored, the remaining sixdefine a regular hexagon and its greatest dimension is disposedvertically. The appropriate arrangement of the six-point exposure systemfor an aperture mask having this aperture array is indicated in FIG. 8.In this figure, the broken-line circle G at the center denotes the placewhere the point light source is located for conventional screening. Itis spaced a distance S from the axis, as explained in discussing FIG. 2,and simulates the green electron gun. For the case at hand, however, nolight source is used at that location; instead a series of six-lightsource are arrayed in a regular hexagonal pattern with the light sourcespositioned at each apex. The separation between light sources is givenon the drawing in terms of the spacing S which, in turn, is related tothe spacing W of the mask apertures as follows:

The major dimension of the hexagonal array of light sources is disposedhorizontally, that is to say, in the direction of horizontal scan andhas a value of 2 3 times S.

Use of the preferred array of six-light sources results in threesimultaneous exposures of each exposed elemental area of the screen asdescribed in relation to FIG. 5. There is an advantage here in that eachsuch area is simultaneously exposed through three apertures of the maskso that if one of the three has an out-ofround imperfection the effectof the defect tends to be suppressed by exposure through other aperturesadmitting light to the same elemental area of the screen. An evengreater benefit of this nature is described and claimed in aconcurrently filed application Ser. No. 179,920 of William Rowe et alassigned to the assignee of the present invention. In Rowe et al theremay be as many as seven, and even more, simultaneous exposures ofselected elemental areas of the screen in process.

It has been stated above that the screen may be printed photographicallyutilizing photosensitive coating materials or, alternatively, it may beprinted electrostatically. The processes are similar in that exposure ofthe coated substrate through the shadow mask causes a change in asurface characteristic to create a latent image of a pattern ofelemental areas that is subsequently developed. Utilizing photosensitiveresists causes the surface phenomenon to be a change in solubility ofthe layer coating the substrate, whereas in electrostatic printing thechange in surface characteristic is a change in a charge pattern. It isnot necessary to burden this disclosure with the details ofelectrostatic screening because it is described in U.S. Pat. No.3,475,169, issued Oct. 28, 1969 in the name of Howard G. Lange andassigned to the assignee of the present invention.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects and, therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

I claim: 1. In the manufacture of shadow mask-type color cathode raytubes having a phosphor screen on an inside surface of a faceplate forthe tube including interlaced patterns of red, blue and green phosphorelements, in the formation of a first of said patterns of phosphorelements, a method comprising:

depositing on the faceplate a coating including phosphor material and apositive photoresist material;

supporting adjacent the faceplate a color selection electrode having apattern of apertures corresponding in geometry to the desired patternsof phosphor elements;

forming on the coating a negative image of said first pattern ofphosphor elements by exposing the coating through said color selectionelectrode to one or more pairs of sources of radiation actinic to saidcoating disposed in a diagonal relationship about a reference positioncorresponding to the first order color centerlocation for said firstpattern of phosphor elements, one source of said pair or pairs beingdisposed at a first order color center location associated with a secondof said patterns of phosphor elements and the other source of said pairor pairs being disposed at a second order color center locationassociated with the third of said patterns; and

developing said coating to produce a pattern of unexposed photoresistareas corresponding to the desired first pattern of said phosphorelements.

2. The method defined by claim 1 wherein the exposure of said coatingthrough said color selection electrode is accomplished using threesimultaneously activated, diagonally related pairs of sources arrangedin a hexagonal array centered about said reference posi- 10 tion.

3. The method defined by claim 2 wherein the time of exposure of saidcoating to said three pairs of sources is such that upon development ofsaid coating said areas of said pattern of unexposed coating areas aresmaller than said apertures in said color selection electrode.

4. The method in accordance with claim 2 in which the apertures of saidcolor-selection electrode define a series of regular hexagonal patternshaving a major dimension in the vertical direction,

and in which said sources are positioned at selected apices of a similarregular hexagonal pattern centered on said reference position but withits major dimension extending in the horizontal direction.

5. The method in accordance with claim 4 in which one of said sources ispositioned at each apex of said hexagonal pattern centered on saidreference position.

6. The method in accordance with claim 5 in which the center-to-centerspacing of adjacent holes in said color-selection electrode is W,

and in which the spacing of adjacent apices of said hexagonal arraycentered on said reference position is approximately 3 times S, where Sis related to the distance p of the light source from said electrode, tothe spacing q of said electrode from said screen and to W as follows:

than said transparent portions of said electrode.

2. The method defined by claim 1 wherein the exposure of said coatingthrough said color selection electrode is accomplished using threesimultaneously activated, diagonally related pairs of sources arrangedin a hexagonal array centered about said reference position.
 3. Themethod defined by claim 2 wherein the time of exposure of said coatingto said three pairs of sources is such that upon development of saidcoating said areas of said pattern of unexposed coating areas aresmaller than said apertures in said color selection electrode.
 4. Themethod in accordance with claim 2 in which the apertures of saidcolor-selection electrode define a series of regular hexagonal patternshaving a major dimension in the vertical direction, and in which saidsources are positioned at selected apices of a similar regular hexagonalpattern centered on said reference position but with its major dimensionextending in the horizontal direction.
 5. The method in accordance withclaim 4 in which one of said sources is positioned at each apex of saidhexagonal pattern centered on said reference position.
 6. The method inaccordance with claim 5 in which the center-to-center spacing ofadjacent holes in said color-selection electrode is W, and in which thespacing of adjacent apices of said hexagonal array centered on saidreference position is approximately Square Root 3 times S, where S isrelated to the distance p of the light source from said electrode, tothe spacing q of said electrode from said screen and to W as follows: 3Sq (p + q) W.
 7. The method in accordance with claim 6 in which saidmajor dimension of said hexagonal pattern of energy sources is equal to2 Square Root 3 times S.
 8. The method in accordance with claim 2 inwhich the exposure interval is of such duration that the portions ofsaid layer that experience a change in surface characteristic overstepone another and reduce said elemental areas of said third set to a sizesmaller in area than said transparent portions of said electrode.